The present invention relates to the measurement and detection of cracks and fissures in test objects particularly under utilization of light conducting fibres which will break in the instance of a crack or fissure; the light conductors being embedded into the test object will break and the crack in the conductor will impede the light transmission through such a light conductor so that the existence of a crack or fissure can be determined and indicated therewith.
A device of the kind to which the invention pertains is disclosed in our U.S. Pat. No. 4,603,252, filed Nov. 7, 1983; (see also our U.S. Pat. No. 4,629,318). The known device uses synthetic fibre material, e.g. glass fibres or quartz fibres, which are integrated in an object made for example of a synthetic material whereby the arrangement is carried out in a particular pattern so that even during the manufacturing of the particular part any irregularities in the material distribution is detectable through local deviation of the embedded light conductors from the normal pattern of initial and/or intended placement. For crack detection light is coupled into one end of the fibres and unimpeded passage of light through all of the fibres is detected through appropriate light detecting means and devices. As soon as a crack appears the light transmission is drastically reduced and the light detector can readily respond to that reduction causing correpondingly a signal which can be indicated. It is a particular feature of this kind of crack detection that once a crack has occurred the signal will remain. Here it has to be observed that a crack may occur as a result of internal tension or when stress is being applied externally tending to pull the part apart. Once the stress or tension has been removed the crack in the part as such may hardly be noticeable any longer but it is still there. On the other hand the interruption in light conduction of the affected fibre will not be restored, even if the broken fibre parts are so to speak pushed back together. Another advantage of this crack detecting device is to be seen in the fact that this kind of monitoring and indication is not prone to corrosion.